Iodixanol (1,3-bis(acetamido)-N,N′-bis[3,5-bis(2,3-dihydroxypropylaminocarbonyl)-2,4,6-triiodophenyl]-2-hydroxypropane) is a non-ionic X-ray contrasting agent produced in large quantities by GE Healthcare in Lindesnes, Norway. The traditional industrial synthesis of iodixanol involves dimerisation of intermediate 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (“Compound A”) as the final synthetic step. See Scheme 1 and U.S. Pat. No. 6,974,882.

Currently, the only dimerisation agent used in an industrial production of iodixanol is epichlorohydrin, shown below.

It is thus desirable to develop alternative dimerisation reagents that are suitable for conversion of Compound A to iodixanol with acceptable yield.